This invention relates to sensing using electromagnetic waves, in particular, but not exclusively, microwaves, for rapid analysis of substances produced by or during a reaction or other procedure, for example.
GB2203553 discloses a gas sensor having a layer of semiconducting organic polymer such as a polypyrrole that can be exposed to a gas to be detected. An alternating electric signal of varying frequency is applied to conductors bridged by the polymer and the change in impedance characteristic of the sensor when exposed to the gas detected by an impedance analyser. A sensor unit may comprise a number of such sensors of different polymers reacting to different gases. The frequency range used is 1 MHz to 500 MHz.
Known as an ‘electronic nose’, the gas sensor can be trained using a neural net to recognize different sets of changes in impedance of an array of sensors in response to different gases. GB2203553 suggests that it may be possible to detect particular gases by investigating changes in the impedance characteristic localised at particular frequencies, but notes that it is difficult to do this on account of noise, opting instead for a comparison system in which differences in the variation of impedance characteristics as compared with a reference gas such as nitrogen are determined over a range of frequencies and in particular, not using frequencies above 500 MHz.
This is clearly complex and time consuming, and it would appear also that the impedance characteristics change with time, on a scale of minutes. The method appears suitable only for gases or vapours, and, more particularly, gases or vapours that react with a semiconducting polymer.